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Protection against indirect effects of lightning of reinforced concrete buildings
E. Bachelier, F. Issac, D. Prost, C. Miry, E. Amador, P. Duquerroy
To cite this version:
E. Bachelier, F. Issac, D. Prost, C. Miry, E. Amador, et al.. Protection against indirect effects of lightning of reinforced concrete buildings. ICLP 2014, Oct 2014, SHANGAI, China. �hal-01083713�
2014 International Conference on Lightning Protection (ICLP), Shanghai, China
Protection against indirect effects of lightning of reinforced concrete buildings
E. Bachelier, F. Issac, D. Prost ONERA – DEMR/CEM
Toulouse, France Elodie.bachelier@onera.fr
C. Miry, E. Amador, P. Duquerroy EDF R&D - LME
Moret sur Loing France Celine.miry@edf.fr
Abstract—This paper presents a methodology to determine the shielding effectiveness of a reinforced concrete building against indirect effects of lightning. Numerical analysis is then used to compute expected magnetic fields inside such a building struck by lightning current injection.
Keywords: Full-wave numerical methods, computational electromagnetic, lightning, shielding effectiveness
I. INTRODUCTION
When a building is struck by lightning, the propagating lightning current induces over-voltages and electromagnetic fields inside its structure. Particularly, the steel mesh of reinforced concrete buildings is often used as part of the structure lightning protection system (LPS) [1-3]. This paper presents a methodology to determine the shielding effectiveness of reinforced concrete buildings against lightning.
The size of the effective mesh representing the reinforced concrete wall is determined as well as the equivalent surface impedance of the walls. These parameters are used to define numerical equivalent models and to determine the expected magnetic field inside a reinforced concrete building struck by lightning.
II. EXPERIMENTAL ANALYSIS
In the case of buildings built with reinforced concrete and under certain conditions, the reinforcing steel is generally a good lightning protection system (LPS). The metal in the concrete is used as a mesh of the lightning protection system and its connection to the ground network provides efficient draining of the injected lightning current. By using appropriate rules of construction one can assume that protection against lightning strokes is effective at the time of the construction.
However, once the building construction is achieved, it is difficult to guarantee that the quality of the LPS will be maintained.
The first part of this article presents a measurement methodology for analyzing and state whether the steel inside the concrete can be used as a lightning protection system or not.
If the resistance of the connection of the steel reinforcement grid to the ground network is rather easy to check by a specific
measurement, it is much more difficult to test the quality of its internal interconnections and, hence, the electric continuity of the steel mesh.
At a first step, we will look at the way to qualify the electrical interconnections of the reinforcement grid. For this, we measure the coupling factor between two current loops in coaxial or coplanar polarizations. This measurement technique is derived from the technique used for the determination of surface impedances of composite materials in aeronautics. We will show that, under certain conditions, it is possible to define an equivalent impedance and an equivalent size of the mesh representing the reinforced concrete wall. This measurement, performed in transmission (loop on both sides of the wall) or reflection (on only one side of the wall), provides a local evaluation of the interconnection.
At a second step, we will analyze the capacity of the reinforced steel to be used as an electromagnetic shield for a lightning strike. For that, it is necessary to stress the whole structure with a lightning type aggression. One technique consists in injecting a high current in the building structure and to measure the internal magnetic field. In a large size building, it is required to put the generator far away from the building. It is often technically difficult to meet this requirement and a good alternative is to put the generator inside the building. In this configuration, the state of stress of the building is complementary to the state of stress of a lightning strike. We will show that, using a low frequency approximation, the transfer function (in meters) Hext/Iint
obtained with internal injection is equivalent to the transfer function Hint/Ilightning.
Finally, we will compare experimental results obtained on a real reinforced concrete structure with numerical simulations, and we will show how to qualify the metal reinforcements as part of the lightning protection system.
III. NUMERICALANALYSIS
First, measurements will be used to define two different numerical models. The first model, so-called “wire model”, will be defined using thin metallic wires with a mesh size
equivalent to the real one. The second model, called thin sheet model, will be defined with homogeneous surface impedance material deduced from the wall measurement. Numerical analysis will be achieved using the two different solvers briefly described below:
- The CST MWS solver [4] based on the TLM method.
- The ALICE solver [5] based on the FDTD method.
Equations are solved in time domain for both methods. Results obtained with both models (wire and thin sheet) and both numerical solvers (CST and ALICE) will be compared with the second step measurements (EM shielding assessment). For simplification, the whole building will not be considered but only the room where the injection is realized. The error associated to this simplification will be quantified in the full paper.
A. Description of the wire model
The building is represented using a regular mesh size of 25 cm. Conductors are modeled using PEC material. The structure is illustrated in Fig.1.
Figure 1 : Wired model
B. Description of the thin sheet model
The building is represented using thin sheet surfaces of 0.1 m thickness and 2000 S/m conductivity. The structure is illustrated in Fig.2.
Figure 2 : Thin sheet model
C. Results
A current similar to the one used in measurement is injected inside the room. It can be represented by the following bi-exponential function: I0.(e-αt - e-βt) with I0 = 750 A, α = 35714 s-1, β = 666667 s-1, illustrated in Fig.3.
Figure 3 : Injected impulse current
Magnetic field is calculated at the three probe locations described in blue in Fig.4.
P1 X
X P 2
X P3
generator 4m
10m
Figure 4 : Building room top view
A comparison of measured and calculated magnetic fields is given in Fig. 5 to Fig. 7 for each probe location.
In Fig. 5, the measured/calculated magnetic field is due to the current flowing inside the room walls. We observe that the wire model does not reproduce a real building behavior.
Indeed, experimental data clearly outline a low-pass filter behavior which is not the case in the basic wire mesh model.
The thin sheet model better matches experimental data.
In Fig. 6 and 7 we measure/calculate the direct emission of the injection cable trough the entrance of the room. Thus, we do not observe low-pass filter behavior due to wall attenuation. In this case, both wire and thin-sheet model are in good agreement with measurements. Differences between calculated and measured magnitudes can be explained by the locations of the probes which can slightly differ in the models and in the real building. Differences between CST and ALICE thin-sheet models may be due to different mesh cell sizes used. This will be investigated for full paper.
Figure 5 : Magnetic field at position P1
Figure 6 : Magnetic field at position P2
Figure 7 : Magnetic field at position P3
IV. EXTRAPOLATION TO AN EXTERNAL INJECTION
The numerical model of the wall is validated and will be used to simulate a complete building struck by a standard lightning waveform [6].
These results will be presented in full paper.
REFERENCES
[1] I.A. Metwally, F.H. Heidler, “Reduction of lightning-induced magnetic fields and voltages inside struck double-layer grid-like shields”, IEEE Trans. On EMC, Vol. 50, NO. 4, Nov 2008.
[2] J. Raimbourg, S. Bazzoli, J. Gazave, O. Peyssoneaux, M. Mardiguian,
“Champ électromagnétique induit par la foudre sur un bâtiment faradisé : modélisation numérique et validation sur une maquette”, CEM 2011, Limoges.
[3] W. Zhang, Z. Zhao, S. Gu, N. Xiang, “Analysis of magnetic fields radiated by lightning strikes to a building”, 7th asia-pacific international conference on lightning, Nov. 2011, China.
[4] CST-Microwave Studio 2012, user’s manual.
[5] E. Bachelier, F. Issac, S. Bertuol, J.P. Parmantier, “Electromagnetic numerical study for optimizing the lightning protection system of the VEGA launching pad”, ICLP 2010, Cagliari, Italy.
[6] Protection against lightning - Part 1 : General principles, standard IEC 62305-1, Dec. 2010.
